Align FXL auto mode on Reflow’s auto-pagination

[JayPanoz]

For reflowable text, we are using two line-lengths to handle the number of columns:

• minimalLineLength: undefined | null | number (of characters)
• optimalLineLength: number (of characters)

This allows to target an optimal number of characters per line, taking various settings into account e.g. font-family, letter-spacing, etc., resulting in a practical width computed from all these settings. Then this number is used to check how many multiple we can fit into the container.

Of course the same logic can’t be applied to FXL, which is very different by nature, but we should have an equivalent that allows to switch from a single page to a two-page spread e.g. a breakpoint, and that should be part of the Preferences.